1. Field of the Invention
This invention relates to improvements in a paint film arrangement coated on the outer panel of an automotive vehicle in order to increase the rust resistance for an edge of steel plate and the brightness of a coated paint film after completion of a finish coat.
2. Description of the Prior Art
In general, a paint film arrangement for an automotive vehicle outer panel includes an electrodeposition paint film for the purpose of rust prevention, an intermediate coat paint film for the purpose of thickening the paint film arrangement, and a finish coat paint film for the purpose of colouring. The automotive vehicle outer panel coated with such a paint film arrangement is required to have good smoothness, gloss and the like. Total evaluation of such characteristics has been usually accomplished by using a so-called PGD value which is measured by a PGD (Portable Gloss and Distinction) meter. This PGD meter is, for example, a PGD-3 type one (produced by a Japan Colour Research Institute in Japan). It is known to those skilled in the art, that the PGD value becomes high as the smoothness and gloss of the finish coating film is higher. Here, the gloss depends on the performance of the finish coat paint film and seems to be an independent function of a finish coat paint. Concerning the smoothness, it is largely affected by the smoothness of under-coat films and therefore the electrodeposition paint film is eagerly required to have a good smoothness.
As a criterion for the smoothness, a so-called center line average height (roughness) Ra is usually used. The center line average height Ra is measured by using a cutoff wavelength of 0.8 mm. The center line average height Ra smaller than 0.3 .mu. is usually used for conventional automotive vehicle electrodeposition paint film from the view point of external appearance. In such a case, an intermediate coat paint is designed to be intended to harden at a shorter time in order to ensure the smoothness on a horizontal plane and to prevent the smoothness from degrading due to run of paint during baking on a vertical plane. In this regard, the usual used intermediate coat paint has a hardening starting time not less than 12 minutes and less than 15 minutes, measured by an Oscillated Pendulum Analysis (OPA) with an oscillated pendulum type viscoelasticity measuring device. Thus, the conventional paint film arrangement is formed by coating the intermediate coat paint having the hardening starting time (by the Oscillated Pendulum Analysis) less than 15 minutes, on the surface of the electrodeposition paint film having a center line average height not larger than 0.3 .mu..
With the above-discussed paint film arrangement, in order to minimize the center line average height of the electrodeposition coat film, it is carried out to increase the flowability of the electrodeposition paint film during melting thereof. However, under the action of surface tension of the molten coat film at an edge portion of the steel plate of the automotive vehicle outer panel, the edge of the steel plate is exposed without the coat film, so that the above-discussed conventional paint film arrangement is inferior in a so-called edge corrosion resistance.
If the edge corrosion resistance is intended to be improved in the conventional paint film arrangement, it may be proposed to increase the viscosity of the electrodeposition paint film in a molten condition. For this purpose, there are proposed a measure to increase the concentration of pigment in the electrodeposition paint (i.e., P(pigment)/B(binder) ratio), a measure to add non-molten type cross-linking resin particles (i.e., a reology control agent) to the electrodeposition paint, and a measure to lower the hardening temperature to suppress flowing of the electrodeposition paint in a molten state under a cross-linking reaction. However, these measures largely degrade the flowability of the electrodeposition paint in a molten state. Assuming that these measures are taken to obtain a good edge corrosion resistance, the coated surface of the electrodeposition paint has a center line average height Ra not less than 0.3 .mu.. Accordingly, in a case in which conventional intermediate and finish coats are formed on the thus formed electrodeposition paint film, the PGD value of the paint film arrangement on a horizontal plane largely lowers, thereby degrading a commercial value of the resultant automotive vehicle.
In view of the above, it has been eagerly desired in an automotive industry to obtain a paint film arrangement exhibiting both high edge corrosion resistance and high external appearance.